Alkaline storage batteries, including nickel-hydrogen storage batteries, are used in a wide range of areas such as hybrid automobile (referred to as “HEV,” hereinafter) applications and industrial applications (e.g., a backup power). Especially in the HEV application, an alkaline storage battery functioning as a main power supply performs a motor driving (discharge) and accumulation (charge) of regenerative power from a generator. Therefore, the state of charge (SOC) of the battery is monitored and controlled. The above state of charge is also referred to as “depth-of-charge”, and is defined, in numerical terms, as 100% when the battery is fully charged and as 0% when the battery is fully discharged.
When the alkaline storage battery that uses nickel hydroxide as a positive electrode active material repeatedly performs a so-called partial charge/discharge cycle in which a full discharge (SOC is almost 0%) or full charge (SOC is almost 100%) is not performed, a phenomenon (referred to as “memory effect” hereinafter) in which the electromotive force with respect to the residual capacity of the alkaline storage battery decreases, and the capacity of the alkaline storage battery decreases occurs. In order to avoid the memory effect, it is desired that charge and discharge are performed in a wide SOC range in the alkaline storage battery.
However, a plurality of alkaline storage batteries are used in a power supply system for the HEV application in which charge/discharge is constantly and instantly performed with a large current. The capacities of the plurality of alkaline storage batteries vary. In order to prevent the alkaline storage battery with the lowest capacity from being overcharged or overdischarged, an upper-limit depth-of-charge (SOCt) and a lower-limit depth-of-charge (SOCb) are provided, the upper-limit depth-of-charge (SOCt) being set for prohibiting charging the battery to a level equal to or higher than its SOC, and the lower-limit depth-of-charge (SOCb) being set for prohibiting discharging the battery to a level equal to or lower than its SOC, and a method is used in which charge/discharge of the battery is controlled between the SOCt and SOCb. More specifically, the SOCt is set at SOC 70% to SOC 90% and the SOCb is set at SOC 10% to SOC 30% generally. The alkaline storage batteries for the HEV application are mainly used in an intermediate SOC region and are repeatedly charged/discharged on the condition that a full charge or full discharge is not performed. For this reason, the memory effect is easy to take place, so a variety of technologies have been proposed in order to resolve the memory effect.
Patent Document 1, for example, proposes a technology which, when the memory effect is detected, resolves the memory effect by bringing an upper limit value or a lower limit value of an SOC control range close to a full charge level or a full discharge level.
Patent Documents 2 and 3 each propose a battery control device that controls to change an upper limit value or a lower limit value of an SOC of charge and discharge of a secondary battery to prevent the occurrence of the memory effect.
However, the battery control device described in above Patent Document 1 brings the upper limit value or lower limit value of the predetermined SOC control range close to a full charge level or a full discharge level, to thereby resolve the memory effect after the memory effect is detected. Thus, a long time to resolve the memory effect is required. Moreover, if the memory effect that has occurred is significant, the above-described process might not be enough to resolve the memory effect.
The technologies disclosed in Patent Documents 2 and 3 repeatedly charge/discharge their batteries evenly in an SOC region within a prescribed range by fluctuating the upper limit value or the lower limit value of the SOC. Therefore, the technologies disclosed in Patent Documents 2 and 3 have the effect of preventing the occurrence of the memory effect. However, it is difficult for these technologies to completely resolve the memory effect because the SOC is not brought close to a full charge level or a full discharge level.                Patent Document 1: Japanese Patent Application Publication No. 2001-69608        Patent Document 2: Japanese Patent Application Publication No. 2004-166350        Patent Document 3: Japanese Patent Application Publication No. 2007-104803        